1. Technical Field of the Invention
The invention relates to a plug unit for connecting capillary tubes, especially for high-performance liquid chromatography. In addition, the invention relates to a connection system consisting of a bushing unit and such a plug unit.
2. Description of Related Art
In chromatography systems, liquids or gases are fed through suitable connection lines between the components of the relevant system. These connection lines, which can be made, for example, from stainless steel, have suitable connection systems, also called fittings, available at their ends to enable creating a tight connection with the connectors of the components.
Such a connection system has already been described in 1975 in U.S. Pat. No. 3,880,452. A capillary tube forming the connection line between two components is here pushed into the capillary tube receptacle opening of a bushing unit or connection unit and fixed in the bushing by means of a threaded fastener that has a central hole for guiding the capillary tube. For sealing, one or more sealing elements that surround the capillary tube in their front end region are pressed into the capillary tube receptacle opening, which runs conically inward, by means of the threaded fastener when the capillary tube and bushing unit are connected.
However, it is disadvantageous that the sealing position is not realized in the plane of the end surface perpendicular to the longitudinal axis of the capillary tube, but is instead offset rearward a certain distance from the end face in the axial direction. In this way, a dead volume is produced that has a disadvantageous effect, especially in high-performance liquid chromatography. In order to be able to guarantee the tightness of such connections for the extremely high pressures used in high-performance liquid chromatography, sealing elements are often used like those described, for example, in U.S. Pat. No. 4,619,473 as prior art (FIG. 2). That publication involves annular sealing elements that are, for the most part, likewise made from stainless steel and that have, in longitudinal section, a conical profile of the outer diameter. Such a sealing element interacts with a conical receptacle opening in the bushing unit, wherein the conical receptacle opening has a larger angle than the sealing element relative to the longitudinal axis of the capillary tube. Here, when the sealing element is pressed into the receptacle opening, an extremely high, radially inward pressure is exerted by means of a threaded fastener onto the front region of the sealing element, so that the sealing position is realized. Through this pressure, however, a deformation of the sealing element and the capillary tube is usually generated, wherein the sealing element is pressed with its annular front edge into the outer periphery of the capillary tube. Such a deformation is undesired, especially because the sealing element is connected to the capillary tube here with a positive interference fit and non-positive-interference-fit connection and the sealing element can no longer be slipped onto the capillary tube easily in the axial direction. If the sealing connection is released and should such a plug element be screwed into another bushing unit, for example because a component of the chromatography system must be replaced, then a tight connection can indeed be produced, but due to tolerances or manufacture-dependent differences in the depth of the receptacle opening, it can no longer be guaranteed that the capillary tube will again apply a force with its end face onto the end face of the line to which it should connect. If the receptacle opening of the bushing unit of the exchanged component is longer in the axial direction than the previously used component, then an undesired dead volume is created. If the receptacle opening for the capillary tube in the exchanged component is shorter in the axial direction than the previously used component, then the capillary tube is actually deformed, possibly damaged, by the pressure of the threaded fastener, and a tight connection is no longer possible under some circumstances. This is because the sealing element mounted on the capillary tube with a positive interference fit and non-positive-interference-fit connection cannot move in the axial direction.
In the case of such a fitting, however, a small dead volume also can be rarely avoided if the end faces of the capillary tube and the line to be connected are directly opposite each other or contact each other, because the sealing position is not located in the region of the end face of the capillary tube or the line to be connected.
In order to be able to compensate for such tolerances or in order to be able to use connection systems of different manufacturers with one and the same plug unit, a self-adjusting plug for high-performance liquid chromatography is described in U.S. Pat. No. 6,494,500, in which the capillary tube is biased axially toward the capillary tube receptacle openings of the bushing unit by means of a spring provided in the threaded fastener. For the seal, however, a replaceable ferrule is used that again has a conical construction in its front region, and to form the seal interacts with an inner wall of the ferrule receptacle opening that has a more pronounced conical construction. Here there is again the risk that the ferrule is “crimped tight” onto the capillary tube, especially when the sealing element is made from a metal, for example, stainless steel.
In addition, in this plug unit it is disadvantageous that a support element for the helical spring must be mounted on the capillary tube, wherein this support element makes the production of such a plug unit more complicated.
For avoiding a dead volume, a unit is known from U.S. Pat. No. 4,083,702 for connecting capillary tubes for gas chromatography in which the capillary tubes are similarly connected butted to each other with their end faces. The capillary tubes are fixed by means of ring elements that have a wedge shape in the longitudinal section and that interact with corresponding, conical recesses in the connector housing. In this case, however, the seal is simpler relative to a connection that is compatible with high-performance liquid chromatography, since the pressures used for gas chromatography are significantly lower, for example, up to 6 bar.
In addition to the already mentioned disadvantages, the known connection systems feature the disadvantage that the risk arises that the sealing element will remain in place in the relevant receptacle opening of the bushing unit when the plug unit is removed and the capillary tube is pulled out from the bushing unit.
In addition, the trend in high-performance liquid chromatography is toward using thinner capillary tubes, since these can be more easily bent to better adapt them to the existing installation conditions. Due to the production process, thinner capillary tubes also feature smaller tolerances in the inner diameter and smaller eccentricity of the inner diameter. To allow using capillary tubes with smaller outer diameters in connection with conventional bushing units or connection units, the tubes are provided at the ends with a sleeve for adapting the outer diameter to the diameter of the thicker, conventional capillary tubes. This represents an additional expense, however, and increases the dead volume of a connection.